Traditional version control helps you backup, track and synchronize files. Distributed version control makes it easy to share changes. Done right, you can get the best of both worlds: simple merging and centralized releases...

This is a very useful description of a development methodology using the git version control system. This does not exactly match the u-boot methodology, but is very close. Koha git motto: Every time a patch falls on the floor, a kitten dies.

Andrew Moore explains in extremely lucid detail how to use git to "commit early and commit often" in a local working git repository, then rearrange and squash the incremental changesets into logical changesets and, ultimately, create the right patches at the right level of granularity and in the right sequence. Priceless!

Philosophy of custodian trees

My idea of a custodian repository is that it is more than just a
working tool for collecting patches and preparing these for merge
into mainline. My idea is instead that these are pretty much
independent incarnations of U-Boot source trees which users (note:
users, not only developers) with specific needs or interests can
refer to.

For example, in my set of mind somebody interested in the latest 85xx
code would clone the 85xx custodian repository, expecting that he
finds there the most current code for this family of processors.
Probably he will never sync himself against mainline, but instead
continue update (pull) from the 85xx custodian repository.

That means, that my idea is that it is the custodian's responsibility
to provide a permanently accessible, consistent view of his
repository to users. When he collects patches, he will - after
sufficient review and testing - decide that these are good enough to
go into his repository. And at certain points we will pull all the
stuff that has been collected there into mainline.

Upstream and downstream trees

The u-boot tree holds
the mainline code and is the reference for official U-Boot releases.
It is the upstream tree of all other trees; custodians of these trees
must submit a pull request to u-boot for their new code to become part
of the mainline and, eventually, of a release.

Therefore, in the instructions below, when upstream is mentioned, it is
u-boot-arm if the downstream tree is an ARM subtree, and u-boot otherwise.

So from then on, when you see ${upstream}, you should replace this with
u-boot-arm if you are an ARM subrepo custodian, or u-boot if not.

Alternatively, you can define a shell environment variable, using either of:

setenv upstream u-boot or setenv upstream u-boot-arm

set upstream=u-boot or set upstream=u-boot-arm

... depending on your actual shell.

How to get a tree

As this process is not automated, the first step is to apply for a tree
either on the mailing list or at info {at} denx.de directly. All we basically
need is the public part of an ssh key, so we can setup authentication for that
tree.

For the following example we assume that Dave Bowman takes up custodianship
of the U-Boot port for the OpenRISC
architecture and provided us with "id_rsa.pub" after he did an "ssh-keygen -t rsa".
According to our conventions, he will get write access to the tree with the account name
"gu-or32", where gu is the prefix for U-Boot Git custodians.

Initial checkout

To start out, after the account has been setup, Dave simply clones the publicly available tree
as usual:

Pushing changes

Being the owner of the tree, after having done commits on it, Dave can push the changes
upstream via ssh and the ssh public key that he provided in the first step, e.g. doing
the following in the repo should Simply Work™ for him.

Similar errors result if Dave tries to clone from
ssh://gu-or32@git.denx.de/u-boot-or32 - he should really use the "normal"
git repository git://git.denx.de/u-boot-or32.git for cloning or
updating his local tree, the ssh (git+ssh) access method can only be used for
pushing.

But I am asked for a password...

If pushing fails because you are asked for a password,
please verify if you really used the public key,
either through ssh-agent or through an explicit statement in your
.ssh/config file.
With the first form you should have
told your ssh-agent about the public key.
Usually its enough to do a
"ssh-add" as long as you don't have multiple keys;
otherwise do an "ssh-add <keyname>" explicitely.

If you get an error-message like "Could not open a connection to your
authentication agent." then you don't have an agent running. Just do
an "ssh-agent bash" and afterwards the key-adding.

You can always check with "ssh-add -L" what keys can be used by the
agent. Be sure that the key you want to use is on the list.

In case this does not help, then please send the output of
"ssh -vvv <user>@git.denx.de" to wd {at} denx.de .

Tips for maintaining custodian trees

master - Changes that the repository owner wishes to be pulled into the master u-boot repository.

next - Hold changesets waiting for the next merge window to open. This can be based above master

testing - Optional: changes that are in work, published on the u-boot email list, but not ready for the mainline u-boot repository yet. When the patch(es) are mature, they can be "cherry-picked" or merged into the master or next branch and pulled from there into the master u-boot repository.

(Of course there may be other branches for local work in progress, but they should not be pushed to the upstream repository).

Only push useful branches to the custodian repository on git.denx.de and delete branches when they become obsolete.

To see what branches are hanging around:

$ git branch -r # only remote branches

$ git branch -a # local and remote branches

To remove an obsolete branch on the master server (in this example, testing-USB):

Rebase the master, testing and any "work in progress" branches to the ${upstream}/master remote branch (which always reflect the One Repo to Rule Them All). $ git checkout master$ git rebase ${upstream}/master$ git checkout next$ git rebase masterYou can and should also keep the optional branches rebased, preferably on master, or on ${upstream}/master.

This allows the upstream custodian to pull the master branch without having any merge conflicts because the downstream custodian (you) will have seen and fixed them already.

Pushing the testing and/or rebased master branches often requires the -f force flag. This is because the state/content/order of the patches in the branch changed due to the rebase operations.

Tip: I do not use the -f flag the first time just in case it isn't needed.

Send a pull request to the email list and to the upstream custodian(s)

Use an email subject line that includes "Pull request:"

Use git request-pull to show what changes will be pulled and includes a convenient reference to the custodian repository (from the git request-pull parameters). $ git request-pull ${upstream}/master git://git.denx.de/u-boot-${subrepo}.git master

Applying patches

The easiest way to apply patches requires them to be in the format of an e-mail message with a descriptive subject and <Signed-off-by> entry. Multiple patch files can be concatenated. To apply such a patch:

$ git am --signoff --whitespace=strip < PATCH_FILE

BEFORE Requesting a Pull

Before you request a pull, make sure there are no merge conflicts with the mainline. See above for the recommended way of pulling the u-boot master repository tip and rebase the master branch.

As part of the rebase, RESOLVE ALL MERGE CONFLICTS

At this point you can request the branch to be merged into the upstream repository. If the upstream has more changes before the merge occurs, repeat the above steps to fix any bit rot in the subrepo master branch.

Pushing

To push changes in a single branch:

$ git push ssh://gu-${subrepo}@git.denx.de/u-boot-${subrepo} <branch>

Using git push --all is not recommended. It pushes all of your local branches to the custodian repository and results in branch pollution.

After the upstream custodian merges your changes

Since you cloned the custodian subrepo tree, not the mainline, you need to explicitly fetch from the upstream.

$ git rebase ${upstream}/master your master is now right above the upstream master.

Fetching changes from a given branch

$ git fetch git://git.denx.de/u-boot-${subrepo} <branch>

Note: this only copies the branch into your repo. You can then merge, rebase, cherry-pick at your leisure.

Nickname the upstream and frequently fetched repositories

Using git remote add u-boot-${repo} git://git.denx.de/${repo}.git allows you to refer to the repo
by its nickname ${repo} in all fetch operations. For instance, having done once
git remote add u-boot git://git.denx.de/u-boot, fetching the u-boot tree is done simply by
git fetch u-boot.